Continuity mistake: Just before Charles sees Elvira, he takes a cigarette from a box and lights it, but when he goes to get a drink, the cigarette is gone.
Revealing mistake: In the cave scene, when Dr. Edelman and Talbot come upon Frankenstein's monster in the mud, there is a skeleton lying beside the monster, which Dr. Edelman identifies as probably Dr. Niemann. The prop is an obvious lab skeleton, as you can very clearly see the circumferential line around the skull cap. Also, if you look closely in several shots, you can make out the hook at the top of the skull where it would hang from a lab stand. (00:24:35)